The isothermal martensitic transformation (IMT) behavior of Ti-20Nb-0.7O (mol %) alloy was investigated via X-ray diffractometry. Isothermal formation of the alpha '' martensite phase was observed, and the activation energy was determined using the time-temperature-transformation diagram coupled with differential scanning calorimetry. IMT in Ti-20Nb-0.7O alloy was explained by nanometer-sized local lattice modulations (nanodomains) and diffusion of oxygen atoms in the I phase. The latter was found to be the primary rate-determining process.